1. Field of the Invention
The present invention relates to an image processing device capable of processing images in accordance with both the resolution of letters and the tone of pictures and, more particularly, to an image processing device capable of processing images including poorly contrasted letters in accordance with the resolution of such letters.
2. Description of the Prior Art
In a conventional image processing device, such as, the one disclosed in Japanese patent application No. 58-3374, a problem exists in that the resolution is considerably reduced for poorly contrasted letter images such as blurred letters. The problem is due to misjudgement of picture images caused by the smallness of the maximum concentration difference in such images.
For example, when a manuscript P consists of a region A containing well contrasted letters and line figures, a region B containing pictures with smooth concentration gradations, and a region C containing poorly contrasted letters, such as blurred letters, as shown in FIG. 1, typical image signal levels in these regions of the manuscript P appear as shown in FIG. 2. The maximum concentration difference .DELTA.Dmax within a certain area such as that corresponding to 4.times.4 matrix formed by 16 picture elements in each of these regions A, B, and C are, in a representation in which the entire range of the concentration is represented by 8 bit (0-255 in decimal, or 0-ff in hexadecimal), .DELTA.Dmax=dd-ff (hex) for the region A, .DELTA.Dmax=10-40 (hex) for the region B, and .DELTA.Dmax =10-40 (hex) for the region C.
Thus, if letters and pictures were distinguished from each other by the following condition; EQU .DELTA.Dmax&gt;Th.fwdarw.letters EQU .DELTA.Dmax.ltoreq.Th.fwdarw.pictures
with the threshold Th equal to 80 (hex), the region A is recognized as letters and the regions B and C are recognized as pictures. As a result, a straightforward binarization with respect to a fixed threshold value is performed in the region A, while a dither process is carried out in the regions B and C, so that the satisfactory results can be obtained with regards to the letters in the region A and the pictures in the region B, but the resolution of the poorly contrasted letters in the region C is considerably reduced because the processing was performed incorrectly.
On the other hand, if the threshold Th was set equal to a smaller value such as 30 (hex) or 10 (hex) so that the poorly contrasted letters in the region C may be identified correctly as letters, then the region B is also identified as letters, and the tone of the pictures in the region B may not be preserved because of this misjudgement.
Thus, according to the prior art image processing device, it is not possible to distinguish the region A of letter and line figure images, the region B of picture images, and the region C of poorly contrasted letter images from each other unambiguously. Consequently, it is not possible to perform image processing in accordance with the particularity of image contained in each region such that both the resolution of letter images and the tone of picture images are respected to a satisfactory degree.
Moreover, according to the prior art image processing device mentioned above, there is another problem is that the image concentration is considerably reduced for bold letter images due to misjudgement of such images as picture images. The problem is caused by the smallness of the maximum concentration difference within the width of such images resulting in appearance of blanking within the width of such images.
For example, when a manuscript P consists of a region A containing well contrasted letters and line figures, a region B containing pictures with smooth concentration gradation, a region C containing poorly contrasted letters such as blurred letters, and a region D containing bold letters with thick widths, as shown in FIG. 3, typical image signal levels in these regions of the manuscript P appear as shown in FIG. 4. The maximum concentration difference .DELTA.Dmax within a certain area such as that corresponding to 4.times.4 matrix formed by 16 picture elements in each of these regions A, B, C, and D are, in a representation in which the entire range of the concentration is represented by 8 bit (0-255 in decimals, or 0-ff in hexadecimal), .DELTA.Dmax=dd-ff (hex) for the region A, .DELTA.Dmax=10-40 (hex) for the region B, .DELTA.Dmax=10-40 (hex) for the region C, and .DELTA.Dmax=0-5 (hex) for the region D.
Thus if letters and pictures were distinguished from each other by the following condition: EQU .DELTA.Dmax&gt;Th.fwdarw.letters EQU .DELTA.Dmax.ltoreq.Th.fwdarw.pictures
with the threshold Th equal to 80 (hex), the region A is recognized as letters and the regions B, C, and D are recognized as pictures. As a result, a straightforward binarization with respect to a fixed threshold value is performed in the region A, while a dither process is carried out in the regions B, C, and D, so that the satisfactory results can be obtained with regards to the letters in the region A and the pictures in the regions B but the resolution of the poorly contrasted letters in the region C is considerably reduced and the image concentration of the bold letters in the region D is considerably reduced to produce blankings because the processing was performed incorrectly in such a manner in which the tone is preserved.
On the other hand, if the threshold Th was set equal to a smaller value such as 3 (hex) or 1 (hex) so that the poorly contrasted letters in the region C and the bold letters in the region D may be identified correctly as letters, then the region B is also identified as letters and the tone of the pictures in the region B may not be preserved because of this misjudgement.
Thus, according to the prior art image processing device, it is not possible to distinguish the region A of letter and line figure images, the region B of picture images, the region C of poorly contrasted letter images, and the region D of bold letter images each other unambiguously. Consequently, it is not possible to perform image processing in accordance with the particularity of image contained in each region such that both the resolution of letter images and the tone of picture images are respected to a satisfactory degree.